1. Field of the Invention
The present invention relates to plasma processing apparatuses and plasma processing methods. More particularly, the present invention relates to a high-performance plasma processing apparatus that includes current-detecting means for measuring a discharge current flowing in the gap between an electrode pair during plasma discharge, that can confirm the uniform plasma treatment of a workpiece, and that can stabilize the effective electric power consumption in a plasma space, and to a plasma processing method using the apparatus.
2. Description of the Related Art
FIG. 7 shows a typical plasma processing apparatus that has been used for plasma processes, such as CVD (chemical vapor deposition), sputtering, dry etching, and ashing. Referring to FIG. 7, the plasma processing apparatus includes a pair of electrodes composed of a plasma excitation electrode 4 for forming a plasma and a susceptor electrode 8 that faces the plasma excitation electrode 4. A workpiece W is placed on the susceptor electrode 8. The plasma excitation electrode 4 is connected to a feeding terminal of an RF generator 1 with an RF feeder 3 and a matching circuit 2A therebetween. The matching circuit 2A matches the impedance between the RF generator 1 and the plasma excitation electrode 4. The matching circuit 2A is accommodated in a conductive chassis 120. Also, the RF feeder 3 and the plasma excitation electrode 4 are enclosed in a conductive housing 21.
RF power generated from the RF generator 1 is supplied to the plasma excitation electrode 4 via the matching circuit 2A and the RF feeder 3. A shower plate 5 with many openings 7 is provided below the plasma excitation electrode (cathode electrode) 4. An annular projection 4a abuts on the shower plate 5. A conductive gas inlet pipe 17 is connected to a space 6 provided between the plasma excitation electrode 4 and the shower plate 5. The gas inlet pipe 17 has an insulating component 17a at the middle thereof to electrically insulate the plasma excitation electrode 4 from the gas supply source. Gas introduced from the gas inlet pipe 17 is supplied through the openings 7 of the shower plate 5 into a chamber 60 surrounded by a chamber wall 10. The upper side of the chamber wall 10 and the circumference of the plasma excitation electrode 4 are hermetically sealed using an insulator 9.
The susceptor electrode 8 carrying the workpiece W, such as a wafer, is arranged in the chamber 60. The susceptor electrode 8 has a common discharge potential and is supported by a shaft 13. The lower portion of the shaft 13 and a chamber bottom 10A are hermetically connected to each other with conductive bellows 11. The chamber 60 is evacuated by an exhaust system (not shown).
Since the susceptor electrode 8 is vertically movable with the shaft 13 and the bellows 11, the distance between the plasma excitation electrode 4 and the susceptor electrode 8 is, therefore, adjustable, while the vacuum is maintained in the chamber 60. The lower portion of the shaft 13 is grounded as a common terminal, and the common terminal of the RF generator 1 is also grounded. The chamber wall 10 and the shaft 13 have the same DC potential.
The matching circuit 2A is disposed between the RF generator 1 and the RF feeder 3. The matching circuit 2A matches the impedance between the RF generator 1 and the plasma excitation electrode 4 in accordance with changes in the plasma within the chamber 60. Thus, the matching circuit 2A generally includes a plurality of passive devices. Specifically, referring to FIG. 8, the matching circuit 2A includes three types of passive devices: a load capacitor 22 (a vacuum variable capacitor), an inductance coil 23, and a tuning capacitor 24 (an air variable capacitor). In the drawing, one inductance coil 23 is connected between the load capacitor 22 and the tuning capacitor 24.
For etching and film deposition using the plasma processing apparatus mentioned above, it is important to maintain processing uniformity. To produce such uniformity, a stabilized plasma must be generated. A conventional plasma processing method for generating a stable plasma includes monitoring of the ground line to control the grounded state and, therefore, to improve the processing characteristics by controlling the number of ions in the plasma, which is dependent on the electrical characteristics, i.e., the current in the ground line. FIG. 9 shows a typical plasma processing method using ground line monitoring.
Referring to FIG. 9, a plasma processing apparatus includes an etching apparatus 101 and a process controller 102. The process controller 102 controls vacuum exhaust and a state of etching gas supply in the etching apparatus 101, and also controls the RF power for generating a plasma and the like. The etching apparatus 101 includes a processing chamber 103 and a stage 105. The processing chamber 103 is vacuum-sealed with a dielectric discharge pipe and microwaves are passed therethrough. The stage 105 is arranged in the lower portion of the processing chamber 103. A semiconductor wafer 104 serving as a sample is placed on and electrically insulated from the stage 105.
A mirror magnetic field is applied between the processing chamber 103 and the semiconductor wafer 104 by a solenoid coil and a permanent magnet (neither are shown). In this state, the processing chamber 103 is evacuated to produce a high vacuum. Then, process gas is introduced at a predetermined gas pressure. Furthermore, the microwaves, which are generated in a magnetron, are introduced into the processing chamber 103 through a waveguide (not shown) and are applied to the plasma excitation electrode (cathode electrode, not shown). A microwave discharge induced plasma state thereby occurs. The resonance between an electronic cyclotron motion and microwaves in the magnetic field induces the microwave discharge.
In the etching apparatus 101, the processing chamber 103 is connected to ground via a variable resistor (current control means) 111 and an ammeter (measurement means) 112. Accordingly, the ammeter 112 outputs a measured value for the ground line in the processing chamber 103. The output terminal of the ammeter 112 is connected to a computer 113. The computer 113 controls the resistance of the variable resistor 111 to a predetermined value based on the current value from the ammeter 112.
In this case, the number of ions in the plasma forms a dependent relationship with the current in the ground line. That is, the ions disappear into the wall of the grounded processing chamber 103 or the surface of parts in the processing chamber 103, and a current thus flows into the ground line. Consequently, the number of disappearing ions, that is, the number of ions in the plasma, can be controlled by controlling the current flowing into the ground line.
In general, the plasma processing apparatus described above may not start discharging until the RF voltage applied to the gap between the plasma excitation electrode 4 and the susceptor electrode 8 is exceeds a discharge starting voltage. Therefore, the RF voltage between discharge electrodes must be monitored at least when discharge starts in order to adjust the RF voltage to a level exceeding the discharge starting voltage. Heretofore, the RF voltage has been adjusted by detecting reflected waves with a directional coupler (not shown) included in the RF generator 1 and by adjusting the level of reflected waves to zero. However, in some cases, even when the level of reflected waves becomes zero by this detection method, discharge does not start. Also, this conventional monitoring method cannot detect the unevenness of discharge current density on an electrode surface, thus inhibiting the uniform plasma treatment of a workpiece.
Similarly to FIG. 8, an RF output is controlled to a predetermined value at the output terminal of the RF generator. For measuring a plasma excitation current, part of a ground line current is introduced into a bypass circuit: hence, part of the overall current, for example, only two to three percent of the overall current is used for the measurement. Thus, the plasma excitation current cannot be accurately measured.
Also, in the above-mentioned method, the bypass circuit includes a resistor for current measurement. Therefore, current loss due to impedance of this bypass circuit occurs. Accordingly, it is impossible to accurately measure the electric power used for plasma excitation.
Furthermore, in the conventional method, an output of the RF generator is adjusted to a predetermined value at a power outlet. Therefore, the power loss in the matching circuit fluctuates due to an increased temperature of a conductor caused by high-frequency current or the like, thus causing a fluctuation in effective power consumption in the plasma space. For example, the increase in temperature of the conductor causes an increase in impedance in an RF power circuit, thus resulting in a decrease in the effective electric power consumption in the plasma space. In addition, in an apparatus including a plurality of continuous plasma chambers, effective electric power consumption undesirably varies depending on the plasma space.
In order to solve the foregoing problems, embodiments of the present invention provide a plasma processing apparatus that includes a current-detecting unit that can measure a discharge current flowing in the gap between an electrode pair during plasma discharge. In alternative embodiments, the plasma processing apparatus includes a current-detecting unit and also can monitor the uniformity of plasma treatment of a workpiece.
The present invention also provides a plasma processing apparatus that can perform stable and uniform plasma treatment by directly and accurately measuring the RF power applied to a plasma excitation electrode (cathode electrode) and by controlling the RF power to a set predetermined value.
In order to solve the above problems, the present invention provides a plasma processing apparatus that includes an electrode pair having a plasma excitation electrode for forming a plasma and a susceptor electrode facing the plasma excitation electrode, a workpiece to be plasma-treated being placed therebetween; a plasma processing chamber accommodating the electrode pair; an RF generator; a feeding path for feeding RF power from the RF generator to the plasma excitation electrode; an impedance matching circuit placed in the feeding path, wherein the impedance matching circuit matches the impedance between the RF generator and the plasma processing chamber; a chassis accommodating the impedance matching circuit, wherein the chassis functions as part of a return path from the susceptor electrode to the RF generator; and a current-detecting unit in the chassis, wherein the current-detecting unit detects an RF current which returns from the susceptor electrode to the RF generator.
While not wishing to be bound by any theory, the present invention is based on the findings that the maximum RF discharge voltage can be obtained when the current flowing in the gap between the electrode pair, that is, in the plasma space, becomes maximum. An indirect but especially effective technique for measuring an RF current flowing in the plasma space involves externally detecting the RF current which returns from the susceptor electrode to the RF generator using a current-detecting unit. The current-detecting unit is provided in the chassis which accommodates the impedance matching circuit. Consequently, if the state of the plasma processing apparatus is set to achieve the maximum current value from the current-detecting unit, the maximum RF discharge voltage can be achieved in the plasma space, thus ensuring that the plasma discharge is started. In the plasma processing apparatus according to the present invention, a high-frequency current flowing in the plasma space can be measured externally (and indirectly) in the return path. Accordingly, the operational status inside the apparatus, such as whether or not a discharge voltage required for starting plasma discharge can be achieved, can be accurately determined externally.
It is preferable that a plurality of the current-detecting units be provided in the chassis, and that the current-detecting units be placed axisymmetrically around the central axis of the chassis.
The presence or absence of drift in the high-frequency current flowing around the chassis can be detected by providing a plurality of the current-detecting units placed axisymmetrically around the central axis of the chassis. If drift is detected around the chassis, the drift may exist in a discharge current flowing in the gap between the electrode pair. The state of the plasma processing apparatus is adjusted to eliminate this drift, thus enabling the drift of the discharge current to be prevented. Thus, greater uniformity in the plasma treatment effect can, therefore, be applied to the workpiece.
Although the expression xe2x80x9caxisymmetricalxe2x80x9d is generally defined as a state in which two points are located on a straight line which is orthogonal to the central axis and at the same distance from the central axis, a state in which a plurality of points are located at the same distance from the central axis on a plain surface which is orthogonal to the central axis and are equally spaced from each other is also included in the present invention.
Preferably, the current-detecting unit includes a slit formed in the chassis and extending along a flow path of the RF current which returns to the RF generator and a magnetic field probe for detecting a magnetic field generated at the slit.
A magnetic field is generated by the high-frequency current at the slit formed on the chassis along the high-frequency current flow path. Since the magnetic field density corresponds to the amount of high-frequency current, the size of the high-frequency current flowing in the plasma space can be measured externally by the size of the magnetic field detected by the magnetic field probe being monitored. It is desirable that the magnetic field probe be placed close to one end of the inside edge of the slit. This is because a high-frequency current mainly flows on the inside surface of the chassis and a magnetic field closer to an edge of the slit has higher density, thus increasing the detection sensitivity.
Preferably, the width of the slit is xcex/100 or less, where xcex represents the wavelength of the RF current.
A slit width exceeding xcex/100 is undesirable because it causes an increase in undesirable emission by the magnetic field and this electromagnetic emission may have adverse effects on the surroundings. The lower limit of the slit width is not particularly limited, as long as the magnetic field probe can be inserted into the slit. In view of this, it is further desirable that the slit width be set to approximately xcex/10,000.
It is preferable that a cross-section of the chassis cut perpendicularly to the central axis thereof is regularly polygonal or circular. It is also preferable that the plasma processing chamber and the susceptor electrode be symmetrical with respect to respective axes of symmetry thereof, and the axes each coincide with the central axis of the chassis.
Generally, uneven plasma processing is caused by the apparatus configuration, the workpiece placement, unevenly distributed plasma generation gas, or the like. Consequently, the apparatus configuration must be considered in order to minimize the unevenness of the processing by reducing drift in the high-frequency current as much as possible. If the cross-section of the chassis is regularly polygonal or circular, the density of the high-frequency current flowing in the sidewalls of the chassis is constant over the circumferential sidewall, thereby reducing or preventing drift in the discharge current flowing in the gap between the electrode pair. Moreover, if the plasma processing chamber and the susceptor electrode are also symmetrical with respect to respective axes of symmetry thereof and the axes each coincide with the central axis of the chassis, a more effective reduction or prevention of drift in the current flowing between the electrode pair is achieved.
A plasma processing apparatus according to the present invention includes a plasma processing chamber having an electrode for exciting a plasma; an RF generator for supplying RF power to the electrode; a matching circuit for impedance matching between the plasma processing chamber and the RF generator, the matching circuit including an input terminal connected to the RF generator with an RF supplier therebetween and an output terminal connected to the electrode with an RF feeder therebetween, wherein a ground potential portion is connected between the input and output terminals; a current-detecting unit for monitoring a current flowing in the RF feeder; a control unit for controlling an output of the RF generator so that a current value detected by the current-detecting unit maintains a predetermined value; and a feedback circuit for feeding back to the RF generator or the matching circuit a control signal from the control unit to adjust the electric power applied to the electrode for exciting the plasma.
By using such a plasma processing apparatus, it becomes possible to directly and accurately measure the RF power applied to the plasma excitation electrode, to control the RF power to predetermined electric power, and to perform stable and uniform plasma treatment.
In the plasma processing apparatus according to the present invention, a current probe may be used as the current-detecting unit.
Therefore, the RF power can be accurately measured by using a simple apparatus.
Also, the plasma processing method according to the present invention is executed by using the above-described apparatus. In the method, electric power applied to the electrode for exciting a plasma is controlled to maintain a predetermined value during a plasma treatment.
More specifically, in the method, a current flowing in an RF feeder and applied to the electrode for exciting the plasma may be monitored by a current-detecting unit. Also, the electric power applied to the electrode for exciting the plasma may be controlled so that the current detected by the current-detecting unit maintains a predetermined value.
By using such a plasma processing method, effective electric power consumption in the plasma space can be kept constant, so that uniform processing can be maintained when an etching process, a film deposition process, a sputtering process, or the like is performed.